A full year has passed since epidemic experts began tracking the H1N1 virus that was also known as swine flu. The pandemic is over for now, but the virus is still active. Both the World Health Organization and the U.S. Centers for Disease Control are reviewing their responses.

Last year at this time, governments around the world intensified efforts to contain a deadly flu virus that killed scores of people in Mexico and quickly spread around the world.

Late last April, World Health Organization Director-General Margaret Chan warned that a pandemic was imminent and the time to prepare for it was short.

"Certain actions should now be undertaken with increased urgency and at an accelerated pace," she stated.

It was the first flu pandemic in more than four decades.

"There was a lot about this new virus that we didn't know," said Dr. Stephen Redd, who headed the H1N1 flu response at the U.S. Centers for Disease Control.

Health officials did not know how deadly the virus would be, who would be most vulnerable, how quickly it would spread and whether it could be contained.

"In the early days, the first few cases in the U.S. had recovered by the time they were diagnosed. But the information we had from Mexico, the first cases were all from people who had died," he recalled.

Health officials had been preparing for a flu epidemic for years, but viruses do not follow a plan.

"In our pandemic work, we were really focusing on the avian influenza, mostly in Asia and Africa, so the assumption that the next pandemic would start overseas, far from the U.S., turned out not to be true," Dr. Redd said.

Dr. Redd says one of the things health officials learned is that the global surveillance system needs to be even more comprehensive. He says communication was a major factor in containing the virus. "Keeping everybody up to speed with what we were doing, what we were learning, what the key issues were. That was extremely important," he stated.

Even high-level officials demonstrated ways to prevent the flu from spreading. Such as washing of hands, coughing into sleeve and throwing away tissues once used.

Flu vaccines are not difficult to produce, but the drug companies found this virus difficult to grow. Dr. Anthony Fauci with the National Institutes of Health expressed frustration with the slowness of the process. "You really can't do anything when you have a virus that doesn't grow well except trying to wiggle it around to get it grow better," he said.

Many experts say there has to be a better way to produce vaccines.

Stephen Morrison is an authority on global health policy at a Washington research center. "Production of vaccines remain highly uncertain and antiquated," Morrison said. He says better technologies need to be developed to speed up production.

Demand drew scores of people to vaccination clinics in the U.S. But there was no plan for distributing a pandemic level vaccine in the U.S. or anywhere.

The new virus led to patterns of death and illness not normally seen in influenza infections.

"In the normal flu season, 90 percent of the deaths occur in people over 65. In this outbreak, close to 90 percent occurred in people under 65," Dr. Redd said.

In 2009, the H1N1 virus killed more than 16,000 people worldwide. Some critics say the World Health Organization overstated the impact of H1N1... that it was really a mild pandemic.

Dr. Redd disagrees. "It had a tremendous impact on children and adults with chronic medical conditions so it would be a mistake to characterize the pandemic as being mild," he said.

Dr. Redd says H1N1 is still a threat and he expects it to be the dominant flu virus in the southern hemisphere in the coming flu season.